Field of the Invention
The present disclosure relates to a lead-free solder alloy, an electronic circuit substrate, and an electronic device.
Discussion of the Background
In the related art, when joining an electronic component to an electronic circuit that is formed on a substrate such as a printed circuit board and silicon wafer, a solder-joining method that uses a solder alloy has been employed. In the solder alloy, lead is typically used. However, use of lead is limited in accordance with RoHS command from the viewpoint of an environmental load. Accordingly, in recent, a solder-joining method with a so-called lead-free solder alloy, which does not contain lead, is generally used.
As the lead-free solder alloy, for example, a Sn—Cu-based solder alloy, a Sn—Ag—Cu-based solder alloy, a Sn—Bi-based solder alloy, a Sn—Zn-based solder alloy, and the like are well known. Among these, in a consumer electronic device that is used in a television, a portable telephone, and the like, and an in-vehicle electronic device that is mounted on a vehicle, Sn-3Ag-0.5Cu solder alloy is widely used.
Solderability of the lead-free solder alloy is slightly inferior to that of a lead-containing solder alloy, but a problem related to the solderability is covered through an improvement of flux or a soldering apparatus. Accordingly, for example, even in the in-vehicle electronic circuit substrate, when the in-vehicle electronic circuit substrate is placed in a relatively warm environment such as a vehicle interior of a vehicle even though a temperature difference is present therein, a significant problem does not occur even in a solder joint that is formed by using the Sn-3Ag-0.5Cu solder alloy.
However, in recent, under a harsh environment such as an engine compartment, direct placing of an engine, and electric/mechanical integration with a motor in which the temperature difference is particularly significant (for example, a temperature difference from −30° C. to 110° C., a temperature difference from −40° C. to 125° C., and a temperature difference from −40° C. to 150° C.), and an vibration load is applied, disposition examination and practical application of an electronic circuit substrate such as an electronic circuit substrate used in an electronic device have been made. In the environment in which the temperature different is very significant, thermal displacement of a solder joint due to a difference in a linear expansion coefficient between an electronic component that is mounted and the substrate, and a stress accompanying the thermal displacement are likely to occur. In addition, repetition of plastic deformation due to the temperature difference is likely to cause a crack in a solder joint, and a stress that is repetitively applied with the passage of time is concentrated to the vicinity of a tip end of the crack, and thus the crack is likely to propagate to a deep portion of the solder joint in a transverse direction. The crack, which significantly propagates as described above, is apt to cause disconnection of electrical connection between the electronic component and an electronic circuit that is formed on the substrate. Particularly, in an environment in which vibration is loaded on the electronic circuit substrate in addition to a great temperature difference, the crack and the propagation thereof are more likely to occur.
Therefore, during an increase of the in-vehicle electronic circuit substrate and the electronic device which are placed in the above-described harsh environment, it is expected that a demand for the Sn—Ag—Cu-based solder alloy capable of exhibiting a sufficient crack propagation suppressing effect increases from now on.
In addition, in the related art, a Ni/Pd/Au-plated component or a Ni/Au-plated component has been widely used in a lead portion of an electronic component such as a quad flat package (QFP) and a small outline package (SOP) which are mounted on the in-vehicle electronic circuit substrate. However, recently, along with a reduction in the cost of the electronic component or substrate downsizing, an electronic component in which the lead portion is substituted with Sn plating or an electronic component including a Sn-plated lower surface electrode has been examined and put into practical use.
During solder-joining, the Sn-plated electronic component is likely to cause mutual diffusion between Sn included in the Sn plating and the solder joint and Cu included in the lead portion or the lower surface electrode. Due to the mutual diffusion, a Cu3Sn layer, which is an intermetallic compound, is greatly grown in a concavo-convex shape in a region (hereinafter, referred to as “vicinity of an interface” in this specification) in the vicinity of an interface between the solder joint, and the lead portion or the lower surface electrode. The Cu3Sn layer has hard and brittle properties, and thus the Cu3Sn layer that is greatly grown in the concavo-convex shape becomes more brittle. Accordingly, under the above-described harsh environment, a crack is more likely to occur in the vicinity of the interface in comparison to the solder joint, and crack propagation instantaneously occurs from the crack that is the origin. Accordingly, electrical short-circuiting is likely to occur.
Accordingly, even in a case of using the electronic component that is not subjected to the Ni/Pd/Au plating or the Ni/Au plating under the harsh environment, it is expected that a demand for the lead-free solder alloy, which is capable of exhibiting the crack propagation suppressing effect in the vicinity of the interface, increases.
Until now, there are disclosed various methods in which an element such as Ag and Bi is added to the Sn—Ag—Cu-based alloy so as to improve strength of the solder joint and thermal fatigue characteristics, thereby suppressing crack propagation in the solder joint (refer to JP 5-228685 A, JP 9-326554 A, JP 2000-190090 A, JP 2000-349433 A, JP 2008-28413 A, WO 2009/011341 A, and JP 2012-81521 A).